Production of biopharmaceuticals such as proteins, peptides, vaccines etc. involves several unit operations where mixtures of fluids (typically aqueous buffers) have to be prepared to high precision and accuracy. Examples of such unit operations are chromatography, where continuous gradients or step gradients are commonly used for elution of columns, buffer exchange, formulation and any operation where a buffer or other fluid is prepared by in-line dilution of a concentrate. For accurate mixing it is necessary to have good control of the flow rates of the fluids to be mixed. In traditional bioprocessing operations this has been achieved with positive displacement pumps (typically piston pumps) of high precision and accuracy, which are stable with time and can optionally be supplemented with feedback loops from accurate in-line flow meters, as described e.g. in US Pat Appl. 2007/0000308.
Due to the high regulatory demands on sanitation and validation of sanitation between batches and campaigns, there is today a strong trend towards bioprocessing plants where all wetted parts are disposable. This means that the sanitation and validation costs can be avoided, which is particularly important for smaller scale bioprocessing plants used e.g. to produce material for clinical trials.
In a set-up for disposable bioprocessing, it is preferred to use pumps where the wetted parts are only disposable tubing, e.g. in peristaltic pumps, or low-cost disposable pump heads, e.g. for specially designed membrane pumps or centrifugal pumps. Such pumps necessarily give lower accuracy and time stability than the traditional piston pumps, which necessitates calibration under the running conditions immediately before use. Peristaltic pumps are often preferred in disposable bioprocessing as they do not require disposable pump heads, but they are particularly prone to drift with time due to movement of the tubing during operation. For calibration, some type of flow meter is necessary and even the flow meter must have disposable wetted parts.
Such flow meters can be made to good precision, e.g. in the case of ultrasound flow meters as described in U.S. Pat. No. 7,673,527. Their accuracy is however limited as pre-calibration of the disposable flow meters is normally avoided to avoid potential contamination with a calibration fluid. This limitation in accuracy does not pose any major difficulties when only one buffer is to be supplied as e.g. in a one-pump chromatography system like similar to the AktaReady™ liquid chromatography system. In mixing systems, the demand for accurate composition control of the fluid mixture is however high, for example in gradient elution of chromatography columns where the separation selectivity depends strongly on the gradient composition.
There is thus a need for a method to provide accurate mixing of fluids in disposable systems using pumps and flow meters with low-cost disposable wetted parts.